1. Field of the Invention
The present invention pertains generally to the field of fasteners and is more particularly directed to a pull-type blind fastener of the type having a pin and sleeve wherein the pin and sleeve are pre-locked, i.e. permanently bonded to each other prior to fastener installation.
2. State of the Prior Art
Pull-type blind fasteners have as their primary components a tubular sleeve and a pin axially movable through the sleeve. The pin has a pin head or equivalent element which engages one end of the sleeve. The opposite or free end of the pin extends from the sleeve and typically has a series of pull grooves which permit the pin to be positively gripped within the nose of an installation tool. The fastener is installed by inserting the sleeve in an installation hole formed in the workpiece to be fastened and the tool nose is placed against the outer end of the sleeve with the grooved pin end within the nose. Upon actuation the installation tool applies pulling relative axially compressive load on the sleeve until the sleeve wall buckles to form a bulb or head on the blind side of the workpiece, which remains secured between the blind-side bulb and a rivet head pre-formed on the exposed end of the sleeve. The excess length of the pin is broken off at a break groove flush with the rivet head.
The shear strength of the installed fastener is attributable in large part to the sleeve, but the remaining pin portion does contribute importantly in this respect and installed fastener shear strength is significantly compromised by separation of the pin from the sleeve. Provision is therefore made for retaining the pin within the installed sleeve either by a friction lock in non-critical applications or by a positive mechanical interlock for critical applications such as aircraft frames. In either case the locking of the pin to the sleeve occurs during fastener installation, prior to which the pin portion which remains after installation is not axially fixed to the sleeve.
Many blind fastener designs are known featuring mechanical pin locking as part of the fastener installation procedure. The mechanical locking action is presently achieved either by a separate locking collar element carried on the pin and forced by the installation tool into a locking groove in the pin as exemplified by U.S. Pat. No. 4,012,984, to Matuscheck and Pratt, No. 4,451,189 among many others, or by compressively flowing sleeve material into a locking groove in the pin to create the required mechanical interlock as in Fry et al U.S. Pat. No. 3,292,482 and in Mil-R-007885 rivets. In both cases the need for a locking groove weakens the pin.
Important criteria in fastener performance are the size and strength of the blind side bulb formed and the degree of hole fill achieved by swelling and radial expansion of the sleeve within the workpiece. Existing fastener designs suffer from significant drawbacks in these respects as illustrated by the following examples selected from some of the most popular aerospace blind fasteners currently in use.
In a first type of currently available blind rivet (meeting MIL-R-7885 and NAS1738,39 specifications) blind head formation is accomplished by either a separate compressible element such as the collapsible barrel of Pratt U.S. Pat. No. 4,541,189 or the integral shear ring used in Matuschek U.S. Pat. No. 4,012,984 among others, the functions of which is to firstly place a compressive load on the blind side sleeve end to initiate fastener hole filling action, and to secondly slide within the exposed blind-side end of the sleeve thereby creating a large bearing blind head. The integral shear ring or separate compressible element yields or collapses axially as necessary to accommodate differences in workpiece thickness within a permissible grip range characteristic of the particular fastener. This yielding allows continued travel of the pin through the sleeve to a point where it engages a mechanical locking element at which point the portion of the pin protruding from the sleeve can be broken off flush with the sleeve head.
Fasteners of this first type perform adequately but suffer from the following disadvantages:
(a) The shear ring integral to the pin or the separate compressible element axially mounted on the pin are tolerance critical and require costly methods of manufacture the expense of which is ultimately borne by the end user.
(b) The fastener installation loads i.e. the magnitude of relative pulling force between the sleeve and the pin of the fastener necessary for blind head formation and to compress the sleeve so that it swells radially to obtain adequate filling action are inordinately high. This results in a need to use dissimilar materials for the sleeve and the pin as exemplified by Gapp, U.S. Pat. No. 3,148,578, the most popular combination of materials being an aluminum sleeve mated with a heat treated, cadmium plated alloy steel pin. By using a stronger pin material than the sleeve material, the pin diameter necessary to axially collapse the sleeve can be kept relatively small in relation to the sleeve cross section. This is desirable because the installed tensile strength of the fastener depends primarily on the sleeve wall thickness which it is therefore desirable to maximize in relation to the pin cross section for a given fastener diameter. Such combination of dissimilar materials carries a weight penalty caused by the relatively heavy steel pin in addition to the corrosion problems associated with dissimilar metals.
A second type of fastener which attempts to overcome the problems of dissimilar materials is disclosed in Orloff U.S. Pat. No. 3,253,495 and such fasteners are available under specifications MS90353/54, MS21140/41, and NAS1919/21. These blind fasteners are characterized in that a variable hardness gradient is created in the sleeve by selectively annealing the blind head formation zone. The annealed sleeve section is softer than the end sections of the sleeve and bulges more readily under axial loading to form the blind head. The selective annealment alone however is not sufficient to allow use of similar pin and sleeve material with a favorable pin/sleeve cross-sectional ratio. These fasteners typically require further weakening of the sleeve by providing an enlarged hole portion in the sleeve so as to reduce the sleeve wall thickness along the annealed section and thereby reduce the installation load necessary to form the blind head in order to make the sleeve operative in combination with pins made of similar material to the sleeve. Sleeves of this type therefore have a stepped inside diameter or a counterbored through hole. This approach allows a relative increase in sleeve wall thickness within the workpiece hole, but the sleeve wall thickness along the blind head formation zone remains relatively thin compared to the pin diameter in that zone. The pin/sleeve fractional percentage ratio of overall fastener cross sectional area along the bulb formation zone is generally 30/70 while within the workpiece hole where the sleeve through-hole is of smaller diameter the same ratio is approximately 50/50. In these fasteners the blind head is formed by a pin head in external abutment with the blind side sleeve end. The pin head is compressively forced against the end of the sleeve, causing the variable hardness gradient section to bulge outwardly at the blind-side sheet line of the workpiece, forming a large bearing blind head. The pin travels axially through the sleeve compressing the blind sleeve end until the pin engages a mechanical locking element which stops the pin with a break groove in alignment with the sleeve head so that the pin breaks off flush with the sleeve head. The performance trade-off in such fasteners is that while the sleeve section of reduced wall thickness yields readily to the compressive force exerted by the pin head, little or no compressive workpiece hole filling action occurs in the sleeve section of heavier wall thickness. Fasteners of this type (MS 90353/54 and MS 21140/41 type fasteners) are therefore generally referred to as blind bolts since no hole filling action is apparent. A variant of this same basic design is found in NAS1919/21 type blind fasteners in that an additional hole filling feature is included and these latter fasteners are therefore considered to be blind rivets. The hole filling feature consists of an enlarged diameter portion on the pin which is forced or dragged through the sleeve along the reduced through-hole section, extruding and expanding radially outwardly the sleeve as the pin is pulled through. This sleeve expansion is not uniform through the workpiece because the enlarged pin portion or plow does not travel or act on the full length of the sleeve and also because no radial sleeve expansion occurs in the thinner walled counterbored section of the sleeve.
Fasteners of this second group meet all desired functional requirements including the use of similar pin and sleeve materials, but retain the following disadvantages:
(a) The stepped inside diameter or counter-bore of the sleeve is tolerance critical, both from a size and concentricity standpoint and can only be formed from the sleeve blind end in the manufacture of the sleeve. The necessity for a thin walled section at the blind end of the sleeve to maintain low blind head formation loads compounds the tolerance criticality of the fastener structure. Failure to maintain the necessary tolerances results in frequent improper blind head formations ("tulip bulbs") which require costly remake or rework activities.
(b) The blind rivet fasteners of the second group variant having the aforementioned additional hole filling feature require greater blind-side clearance because the pin extends out of the sleeve to a greater extent due to the enlarged extrusion element provided on the pin. Also, the stepped inside diameter of the sleeve causes variable hole fill so that the hole fill is not uniform through the full length of the installation hole in the workpiece. Such variable hole fill can also head to improper blind head formations.
(c) The blind side heads formed by these rivets are particularly weak because their formation load must be safely below any other fastener installation load, i.e. the mechanical lock engagement loads between the pin and the sleeve (with or without frictional forces therebetween taken into account) and the pin breakoff load at the breakneck.
The outer sleeve diameter of a given fastener, and therefore the combined sleeve wall and pin cross sectional areas, is selected according to the requirements of the particular application and is set by the installation hole in the workpiece. In other words, the overal fastener or sleeve diameter is largely determined by factors external to the fastener itself. The relative proportions of the overall fastener cross-section attributable to the sleeve wall and to the pin however are a function of fastener design. For a given outer sleeve diameter, it is generally desirable to maximize the sleeve wall thickness in order to maximize installed fastener tensile strength. However, sleeve wall thickness can only be increased at the expense of a reduction in the pin diameter. As the sleeve wall thickness is increased, it offers proportionally greater resistance to axial compression by the pin and thus a greater installation load is needed between the pin and the sleeve. As the pin diameter is decreased to obtain a heavier sleeve wall, the strength of the pin is proportionally diminished until a pin diameter is reached where the pin would break prematurely under the required installation load. In prior art fasteners pin strength has been compromised by the need to provide various features, structures and grooves such as locking grooves, steps, etc. on the pin, of which the pin break groove must necessarily be the smallest so as to prevent the pin from breaking at an improper point. At the same time, the pin break groove diameter sets an upper limit for the pulling force which can be applied to the pin during fastener installation, which in turn limits sleeve wall thickness, the size and strength of the blind heads, and the degree and extent of hole fill achieved.
Further, because of the internal steps, counterbores and other features required on sleeves of prior art fasteners it is conventional in the art to fabricate the sleeves from solid wire stock by a progressive cold forming process which makes it very difficult to maintain close tolerances in uniformity of sleeve wall thickness, concentricity of the sleeve bore and squareness of the blind side sleeve end surface against which bears the pin head. All these are factors which affect the uniformity and reliability of the joints obtained with the fasteners and close control over the same is highly desirable.
A continuing need therefore exists for fasteners having superior pin strength in relation to maximum pin diameter so that heavier sleeves can be compressed for a given pin diameter resulting in stronger and larger blind-side heads, improved and more uniform hole fill, and greater installed fastener strength while using similar pin and sleeve materials. Such an improved fastener should be simple and inexpensive to manufacture with a minimum number of parts. It is further desirable to make the fastener parts from raw materials and by manufacturing processes which minimize tolerance criticality problems.